1. Field of the Invention
The present invention relates to a flat panel display device and more particularly, to an active matrix electro luminescence display (ELD) device having thin film transistors and manufacturing method for the same.
2. Discussion of the Related Art
As the information age has been evolved rapidly, the necessity for flat panel display, which has advantages such as thinness, lightweight and lower power consumption, has been increased. Accordingly, various flat panel display devices such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission display devices and electro luminescence display (ELD) devices have been researched. The electro luminescence display (ELD) device makes use of electro luminescence phenomenon in which light is generated when an electric field of certain intensity is applied to a fluorescent substance. The electro luminescence display (ELD) devices can be classified into inorganic electro luminescence display (ELD) devices and organic electro luminescent display (ELD) devices depending on the source that excites careers. Attention recently has focused on the organic electro luminescent display (ELD) device as a displaying device for natural colors because it can display every colors in the visible range and has a high brightness and a low action voltage. In addition, because the organic electro luminescence display (ELD) device is self-luminescent, it has a high contrast ratio and is suitable for an ultra-thin type display device. Moreover, because it has a simple manufacturing process, the degree of environmental contamination is relatively low. Besides, the organic electro luminescence display (ELD) device has a few microseconds (ps) response time so that it is suitable for displaying moving images. The organic electro luminescence display (ELD) device has no limit in a viewing angle and is stable in low temperature conditions. Because it is driven with a relatively low voltage between 5V and 15V, manufacturing and design of a driving circuit is easy. A structure of the organic electro luminescent display (ELD) device is similar to that of the inorganic electro luminescence display (ELD) device but the light-emitting theory is different from that of the inorganic electro luminescence display (ELD) device. That is, the organic electro luminescence display (ELD) device emits light by recombination of an electron and a hole and thus it is referred to as an organic light emitting diode (OLED). Recently, an active matrix type display in which a plurality of pixels is arranged in a matrix form and a thin film transistor is connected thereto has been widely applied to the flat panel display devices. The active matrix type display is also applied to the organic electro luminescent display (ELD) device and this is transferred to as an active matrix organic electro luminescent display (ELD) device.
FIG. 1 is a circuit diagram illustrating a pixel of a related art active matrix organic electro luminescent display device. In FIG. 1, a pixel of the active matrix organic electro luminescent display device has a switching thin film transistor 4, a driving thin film transistor 5, a storage capacitor 6 and a light emitting diode (LED) 7. The switching thin film transistor 4 and the driving thin film transistor 5 are formed of p type polycrystalline silicon thin film transistor. A gate electrode of the switching thin film transistor 4 is connected to the gate lane 1 and a source electrode is connected to the data line 2. A drain electrode of the switching thin film transistor 4 is connected to a gate electrode of the driving thin, film transistor 5 and a drain electrode of the driving thin film transistor 5 is connected to an anode electrode of the light emitting diode (LED) 7. A source electrode of the driving thin film transistor 5 is connected to a power line 3 and a cathode electrode of the light emitting diode (LED) 7 is grounded to earth. A storage capacitor 6 is connected to the gate electrode and the source electrode of the driving thin film resistor 5. If a signal is applied to the gate line 1, the switching thin film transistor 4 is turned on. If a signal of the data line 2 is applied to the gate electrode of the driving thin film transistor 5, the driving thin film transistor 5 is turned on and thus the light emitting diode (LED) 7 emits light. The storage capacitor 6 serves to keep a gate voltage of the driving thin film transistor 5 constant while the switching thin film transistor 4 is turned off.
FIG. 2 is a cross-sectional view of the related art active matrix organic electro luminescent display device. In FIG. 2, a polycrystalline silicon layer 11, 12, 13, 14, 15 and 16 having an island shape is formed on a substrate 10. The polycrystalline silicon layer 11, 12, 13, 14, 15 and 16 is divided into an active layer 111 and 14 of the thin film transistor and source and drain regions 12, 13, 15 and 16 on which impurities doped. A gate insulating layer 20 is formed on the polycrystalline silicon layer 11, 12, 13, 14, 15 and 16. First and second gate electrodes 21 and 22 are formed on the gate insulating layer 20 over the active layer 11 and 14. An inter layer 30 is then formed on the first and second gate electrodes 21 and 22. The inter layer 30 and the gate insulating layer 20 has first, second and third contact holes 30a, 30b and 30c that respectively exposes a portion of a first source region 12, a second drain region 15 and a second source region 16. A first source electrode 41, a second drain electrode 42 and a second source electrode 43 are formed of conductive metal material on the inter layer 30. The first source electrode 41 is connected to the first source region 12 through the first contact hole 30a, the second drain electrode 42 to the second drain region 15 through the second contact hole 30b and the second source electrode 43 to the second source region 16 through the third contact hole 30c. A first drain electrode (not shown in the figure) is further formed of same material as the first source electrode 41 over the first drain region 13 and connected to the first drain region 13. The first drain electrode (not shown) is also connected to the second gate electrode 22. The second source electrode 43 is connected to the power line 3 of FIG. 3 and it may be extended from the power line 3 of FIG. 1 or be a portion of the power line 3 of FIG. 1. A portion of the second source electrode 43 is overlapped with the second gate electrode 22 and thus forms the storage capacitor Cst. The first gate electrode 21, the first source electrode 41 and the first drain electrode (not shown) constitutes the switching thin film transistor 4 of FIG. 1 and the second gate electrode 22, the second source electrode 43 and the second drain electrode 42 constitutes the driving thin film transistor 5 of FIG. 1. A first passivation layer 50 is formed on the first source electrode 41, the second drain electrode 42 and the second source electrode 43 and the first passivation layer 50 has a fourth contact hole 50a that exposes a portion of the second drain electrode 42. An anode electrode 60 is formed of transparent conductive material on the first passivation layer 50 and the anode electrode 60 is connected to the second drain electrode 42 through the fourth contact hole 50a. A second passivation layer 70 is then formed on the anode electrode 60 and the second passivation layer 70 has a bank 71 that exposes the anode electrode 60.
FIGS. 3A to 3G are cross-sectional views illustrating a fabricating sequence of the related art active matrix organic electro luminescent display device. In FIG. 3A, a semiconductor layer 17 and 18 are formed by forming a polycrystalline silicon layer and patterning it with a first mask on the substrate 10. A buffer layer (not shown) may further be formed of material such as silicon oxide (SiO2) between the substrate 10 and the semiconductor layer 17 and 18.
In FIG. 3B, the gate insulating layer 20 is formed on the semiconductor layer 17 and 18. The fast and second gate electrodes 21 and 22 are formed by depositing material such as metal and patterning it with a second mask. The active layers 11 and 14 and the source and drain regions 12, 13, 15 and 16 are formed by doping impurities into the semiconductor layer 17 and 18 of FIG. 3A using the first and second gate electrodes 21 and 22 as a mask. The impurities are not doped into the active layer 11 and 14.
In FIG. 3C, the inter layer 30 is formed on the first and second gate electrodes 21 and 22 and the first, second and third contact holes 30a, 30b and 30c are formed by patterning the inter layer 30 with a third mask. The first contact hole 30a exposes the first source region 12, the second contact hole 30b the second drain region 15 and the third contact hole 30c the second source region 16.
In FIG. 3D, the first source electrode 41, the second drain electrode 42 and the second source electrode 43 are formed by depositing conductive material such as metal on the inter layer 30 and patterning it with a fourth mask. The first source electrode 41 is connected to the first source region 12 through the first intact hole 30a, the second drain electrode 42 to the second drain region 15 through the second contact hole 30b and the second source electrode 43 to the second source region 16 through the third contact hole 34c. The second source electrode 43 forms the storage capacitor Cst by overlapping with the second gate electrode 22. In addition, the first drain electrode (not shown) and the power line (not shown) are formed at this time. The first drain electrode is connected to the first drain region 13 and the second gate electrode 22, and the power line is connected to the second source electrode 43.
In FIG. 3E, the first passivation layer 50 is formed on the first source electrode 41, the second drain and source electrodes 42 and 43. The fourth contact hole 50a, which exposes a portion of the second drain electrode 42, is formed by patterning the first passivation layer 50 with a fifth mask.
In FIG. 3F, the anode electrode 60, which contacts the second drain electrode 42 through the fourth contact hole 50a, is formed by depositing transparent conductive material on the first passivation layer 50 and patterning it with a sixth mask. The anode electrode 60 becomes a pixel electrode of the active matrix organic electro luminescent display device.
In FIG. 3G, the second passivation layer 70 is formed on the anode electrode 60 and the bank 71 is formed in the second passivation layer 70 by patterning the second passivation layer 70 with a seventh mask. After forming an array substrate of the active matrix organic electro luminescent display device as stated above, the active matrix organic electro luminescent display device is completed by further forming organic luminescent layer over the bank 71 and forming a cathode electrode thereon.
The storage capacitor Cst is for keeping a gate driving voltage of the driving thin film transistor stable and is an important element for displaying high quality images by restraining a pixel voltage fluctuation that is induced by a kick-back voltage of the switching thin film transistor and a leakage current. A storage capacitance of the storage capacitor is proportional to an area of electrodes and a dielectric constant of a dielectric substance between two electrodes, and inverse proportional to a distance between the two electrodes, i.e., a thickness of the dielectric substance. The inter layer 30 is usually formed of silicon oxide (SiO2) and its thickness is about 7,000 Å (angstrom). If the thickness of the inter layer 30 is small, the power line becomes winding and thus electric resistance is increased. Besides, the power line may be broken. Accordingly, there is a limit in reducing the thickness of the inter layer 30. If the area of the electrode of the storage capacitor is increased, it results in a decrease of an area of the pixel electrode, i.e., the anode electrode 60, and thus an aperture ratio is decreased. An active matrix organic electro luminescent display device has been suggested to overcome this problem and it is illustrated in FIG. 4. FIG. 4 is a cross-sectional view of other related art active matrix organic electro luminescent display device. Because the active matrix organic electro luminescent display device of FIG. 4 has a similar structure with that of the above mentioned one of FIG. 2 except a storage capacitor portion, a detailed explanation on a same structure will be omitted. In FIG. 4, a first inter layer 31 is formed on the first and second gate electrodes 21 and 22. A capacitor electrode 80 that is overlapped with the second gate electrode 22 is formed on the first inter layer 31. A second inter layer 32 is formed on the capacitor electrode 80. The second inter layer 32 together with the first inter layer 31 and the gate insulating layer 20 has first, second, third and fourth contact holes 32a, 32b, 32c and 32d. The first contact hole 32a exposes a portion of the first source region 12, the second contact hole 32b the second drain region 15, the third contact hole 32c the capacitor electrode 80 and the fourth contact hole 32d the second source region 16. The first source electrode 41, the second drain electrode 42 and the second source electrode 43 are formed on the second inter layer 32. The first source electrode 41 is connected to the first source region 12 through the first contact hole 32a and the second drain electrode 42 is connected to the second drain region 15 through the second contact hole 32b. The second source electrode 43 is connected to the capacitor electrode 80 and the second source region 16 respectively through the third and fourth contact holes 32c and 32d. The first and second inter layers 31 and 32 are usually formed of silicon oxide (SiO2). A thickness of the first inter layer 31 is about 3,000 Å (angstrom) and a thickness of the second inter layer 32 is about 4,000 Å (angstrom). A summation of the thickness of the first inter layer 31 and the thickness of the second inter layer 32 has a similar value with that of the inter layer 30 of FIG. 2. Because the capacitor electrode 80 is formed between the first and second inter layers 31 and 32 and the capacitor electrode 80 and the second gate electrode 22 form the storage capacitor, a thickness of the dielectric substance is reduced and thus the storage capacitance of the storage capacitor can be increased. However, this active matrix organic electro luminescent display device is manufactured by iterating a photolithographic masking process several times. Because the photolithographic masking process includes many minor processes such as cleaning, deposition of the photoresist layer, exposure, development and etching, manufacturing time and cost depends on the number of masks needed. That is, if only one mask can be omitted for the total manufacturing process, the manufacturing time and cost can be greatly saved. The active matrix organic electro luminescent display device of FIG. 2 needs seven masks and the active matrix organic electro luminescent display device needs 8 masks because the capacitor electrode 80 must be formed of separate metal material. Accordingly, the manufacturing time and cost are increased according to the related arts.